Issue Date: Nov. 17, 2003 | Issue 47 | Volume 75, Posted On: 11/14/2003

Reprinted with permission from Feedstuffs Magazine

Grazing ruminants require free-choice minerals

By LEE R. McDOWELL

For many classes of livestock, including swine, poultry, feedlot cattle and dairy cows, mineral supplements are incorporated into concentrate diets, which generally ensures that animals receive required minerals.

However, for grazing livestock to which concentrate feeds cannot be fed economically, it is necessary to rely on both indirect and direct methods of providing minerals. Self-feeding of free-choice mineral supplements are widely used for grazing livestock.

Direct administration of minerals to livestock in water, mineral licks, mixtures and drenches, ruminal preparations and injections is generally the most economical. Benefits and disadvantage of mineral supplementation methods have been presented (McDowell, 1996, 1997, 2003). An excellent review on providing minerals to ruminants via intraruminal pellets, large intraruminal boluses, intraruminal devices, injectable supplements and copper oxide needles has been prepared by Judson (1996).

Free-choice supplementation

Voluntary consumption of individual minerals or mineral mixtures by animals is referred to as free-choice or free-access feeding. This practice of feeding minerals free-choice to ruminants has been used for many years to supply needed minerals but is often based on the erroneous assumption that the animal knows which minerals are needed and how much of each mineral is required.

Early reports from South African researchers (Theiler et al., 1924; Green, 1925) described phosphorus-deficient cattle with depraved appetites chewing on bones. Since bones were a good source of phosphorus, the belief was held that animals have the ability to select feeds that contain minerals lacking in the diet. Additional early studies with ruminants fed phosphorus-deficient diets indicated that cows and lambs may consume sufficient phosphorus free-choice to meet their requirements (Becker et al., 1933; Bohstedt, 1957). Becker and co-workers (1933) further showed that serum phosphorus was increased to normal by consumption of bone meal, and the greater the intake of phosphorus from feed sources, the less bone meal was eaten.

Arnold (1964) stated that much evidence in the literature shows that most mammals exhibit little nutritional wisdom and that animals will select a palatable but poor-quality diet in preference to an unpalatable, nutritious diet, even to the point of death. Gordon et al. (1954) had earlier measured the preferences of phosphorus-deficient cattle and sheep for supplemental calcium carbonate alone or combined with an equal part of dicalcium phosphate. The animals failed to consume enough of the phosphorus-containing supplement to prevent aphosphorosis.

Under conditions of low calcium or phosphorus intake, lactating dairy cows did not consume enough dicalcium phosphate free-choice to meet requirements or to correct the deficiencies (Coppock et al., 1972, 1976). Therefore, it was concluded that lactating dairy cows had no, or only very limited, appetite for calcium or phosphorus. Burghardi et al. (1982) reported that although consumption of free-choice calcium carbonate was greater for lambs fed calcium-deficient diets, daily gains and feed efficiencies were less than that of control animals.

Another approach to providing free-choice minerals is the use of a cafeteria-style mineral feeder, which offers the animal a choice of as many as 10 or more minerals. Dairy cows did not consume sufficient amounts of cafeteria-style minerals to meet requirements and acceptability rather than appetite or craving for minerals influenced free-choice consumption (Hutjens and Young, 1976; Muller et al., 1977). Maller (1967) presented evidence that domestication has produced an animal that is more responsive to the sensory qualities of feed than to nutritive value. Thus, the ability to select needed nutrients may have been lost through domestication.

Animals that did not receive concentrates are less likely to receive an adequate mineral supply; free-choice mineral mixtures provided to grazing livestock are much less palatable than are concentrates and are often consumed irregularly. Intake must be adequate to meet mineral deficiencies in forages.

The average daily intake of free-choice mineral mixtures by grazing livestock is highly variable (McDowell, 1985). Coppock et al. (1972) measured individual daily consumption of dicalcium phosphate by lactating dairy cows and found individual variation to be large, with a range from zero to more than 1,000 g per head daily.

Little information is available on the individual intake of free-choice mineral supplements. Tait et al. (1992) developed innovative research using a computer system to electronically monitor individual mineral consumption for grazing cattle. The system is based on electronic animal identification and one or more weatherproof electronic scales located at feed stations connected to a computer. The software program identifies and records individual animals, time and duration of visits and quantity of supplement consumed.

Using this equipment with grazing Holstein steers (averaging 350 kg) in a 3.25-hectare pasture, Tait et al. reported consumption of a mineral mixture to range between 60 and 330 g per day, with 65% of steers consuming between 100 and 250 g per day. The average number of daily visits to the station was three per animal, and it was most interesting that a high proportion of visits were in the late evening between 8:00 and 11:00 p.m. These mineral consumptions are quite high compared to cattle intakes on extensive grazing systems where animals travel further to obtain supplements.

Factors that affect the consumption of mineral mixtures have been listed by Cunha et al. (1964), Coppock (1970) and McDowell (1997; 2003).

Soil fertility and forage type consumed. Usually, the higher the level of soil fertility, the lower the consumption of minerals. Barrows (1977) reported that for cattle, salt, calcium, phosphorus and magnesium each appeared to be consumed in relation to the content of the particular element in the grass. A number of reports have shown that cattle on native range consume more mineral supplement than those cattle on improved pastures. Cattle on low-quality or overgrazed pastures consume more mineral supplements.

Season of year. Season of the year affects mineral intake (Cunha, 1983), which is often greatest during the winter or dry season when forages stop growing, lose green color and become high in fiber and lignin and low in digestibility and mineral availability. As plants mature, mineral content declines (McDowell, 1985). Mineral supplement intake is lower when forage quality and quantity is optimal. Under drought conditions, mineral supplement intake is increased to counteract the low mineral availability in the forage and the low level of forage intake due to its reduced palatability (Cunha, 1983).

Available energy-protein supplements. The kind and level of protein-energy supplementation influences mineral supplement intake. Protein and energy supplements that also provide minerals will decrease both the need and desire for free-choice minerals. Weber et al. (1992) reported a wide day-to-day variability in free-choice mineralized salt and protein block consumption by British-bred beef cows. Variation was much greater for salt-type blocks than for the softer, protein-type blocks, with several cows consuming none of the salt-type blocks for periods of several weeks.

Individual requirements. Growth rate, percentage of newborn and milk production influence mineral needs. Added requirements of gestation and lactation increase mineral needs and, thereby, consumption. The higher the level of productivity, the more important an adequate level of mineral intake. Barrows (1977) reported that mineral consumption tended to decline as cows increased in age.

Salt content of drinking water. Naturally high salt concentration of drinking water decreases mineral supplement intake. Livestock have a natural craving for salt. However, if that desire is fulfilled from drinking water high in salt, grazing livestock will consume less or none of a free-choice mineral mixture based on salt. Where naturally occurring salt content of water is high, mineral supplements cannot be based on salt and should be reformulated with other palatability stimulators such as cottonseed meal and molasses.

Palatability of mineral mixture. As previously mentioned, ruminants have no particular desire for the majority of minerals, with the exception of common salt. In a review on salt appetite, Denton (1967) noted that all mammals have the ability to taste salt, and there is a universal liking for it. Becker et al. (1944) noted that the attitude of cattle toward salt in a mineral supplement is inversely related to the amount of salt present in feeds and water. Common salt, because of its palatability, is a valuable carrier of other minerals. If mixtures contain 30-40% salt or more, they are generally consumed on a free-choice basis in sufficient quantities to supply supplementary needs of other minerals.

Many reports testify to the beneficial effects of bone meal in free-choice supplements. Processing methods for bone meal and other supplements affect both the nutritive value of the products and also palatability and, consequently, consumption. Improperly processed bone meals can emit an unpleasant odor. Also, the danger of botulism, foot and mouth disease and other disease conditions can be transmitted from inadequately processed bone meal.

A relatively palatable phosphorus source other than bone meal is monosodium phosphate. Coppock et al. (1972) reported that dicalcium phosphate was preferred to defluorinated phosphate by dairy cattle fed three different diets. Cattle preferred an acid supplement (pH 3.5), such as dicalcium phosphate, to an alkaline supplement (pH 8.5), such as defluorinated phosphate.

When magnesium-deficient cattle are provided with a free-choice supply of supplementary magnesium, such as magnesium oxide, they will die of grass tetany rather than consume this unpalatable source of magnesium. However, when even high concentrations of magnesium oxide (e.g., 25%) are combined with palatable ingredients, grass tetany is prevented.

Palatability and appetite stimulators such as cottonseed meal, dried molasses, dried yeast culture and fat help achieve more uniform, herd-wide consumption. Some of these products not only give the supplement a dust-free, moist and free-flowing character, but also provide energy and protein. Ingredients that increase palatability must be used in moderation, or they will cause overconsumption.

Availability of fresh mineral supplies. Previous diet or access to mineral supplements affects short-term consumption of minerals. When animals are not allowed access to minerals for long periods of time, they may become so voracious that they injure each other in attempting to reach salt. Under these conditions, they will consume 2-20 times the normal daily quantities of minerals until their appetite is satisfied. By overindulging, they may suffer salt poisoning (treatment: access to water).

The choice of palatability or appetite stimulators is important when considering the keeping value of a supplement. Corn meal is a good appetite stimulator when included in a mineral mixture but it ferments more easily than a proteinaceous product such as cottonseed meal. The use of 20-40% salt prevents molding and blowing.

Physical form of minerals. In observations of cattle, mineral consumption is often a minimum of 10-20% less when provided in block versus loose form. Intakes of loose salt-based supplements by ruminants are substantially higher than where the same material is offered in a compressed form (mineral block).

Mineral blocks can be developed on the basis of degree of hardness to take into consideration rainfall, humidity and other environmental conditions. Rain will dissolve too soft of a block causing mineral losses, and yet livestock experience difficulty consuming enough of a hard block to fulfill mineral requirements. If the animals remain only a limited time in the vicinity of mineral blocks, then excessive block hardness will result in reduced mineral consumption. Providing a supplement in block form has the advantages of convenience and much greater resistance to rain and dew. Also, the control of excessive intakes by the use of blocks may be a significant advantage.

Exposure time, previous experience and social interactions. Livestock exposed to new feeds often exhibit neophobia, or a cautious sampling or rejection of the feed that is not related to palatability (Launchbaugh, 1995). The acceptance and degree of preference by grazing animals for a specific supplement is likely to depend on recognition by the animal of the supplement as a potential foodstuff, prior experience of the animal with the same or similar supplements, social interactions and the degree of preference of the animal for the supplement relative to available forages (Provenza, 1996; Dixon et al., 2001).

Experience, age and social interactions will influence supplement consumption. Individual supplement intake variation usually decreases with time, as animals progress through the neophobic eating pattern found with unfamiliar supplements (Bowman and Sowell, 1997). Total time spent consuming supplement for inexperienced animals was lower for the two-year-old than for the three-year-old cows (Sowell et al., 1995).

Social interactions play an important role in supplement consumption by cattle and sheep. Dominant animals often consume large amounts of supplement and prevent other animals from consuming desired levels. It may be possible to change dominance patterns by altering feeder design (Bowman and Sowell, 1997). However, inexperienced sheep commonly increase supplement intake in the presence of more experienced sheep (Foot et al., 1973).

Biological availability

General considerations. There is considerable difference in the availability of mineral elements provided from different sources. A chemical analysis of a mineral element in a feed or mineral supplement is not the same as availability of an element for animals. That portion of the mineral that can be used by the animal to meet its bodily needs is biologically available. A review of bioavailability of the macrominerals and seven microelements is available (Ammerman et al., 1995). These variations in bioavailability of sources must be taken into consideration when evaluating or formulating a mineral supplement.

The differences between a chemical analysis and bioavailability for animals may be seen in phytin-phosphorus. Phytin consists of myo-inositol, which combines with and hinders intestinal absorption of phosphorus and other minerals including iron, manganese and zinc. Probably no more than about 50% of the phosphorus in plant feeds is available to the pig (Cunha, 1977), but ruminants utilize phytin-phosphorus quite satisfactorily (McDowell, 2000).

Chemical and physical forms of mineral elements affect their availability for animals. Iron oxide is virtually unavailable for animals, compared to high availability for ferrous sulfate. Likewise, copper oxide is unavailable as included in a regular diet or free-choice mixture. However, copper oxide as copper needles is available due to the slow release and acid conditions of the abomasum. Sulfur from potassium or magnesium sulfate is highly available when compared to the low bioavailability of calcium sulfate.

Calculations are required to account for both the amount of element in mineral salts and its bioavailability. For example, copper sulfate contains about 25% copper and about 53% copper carbonate. Therefore, it takes about twice as much copper sulfate to provide the same amount of the elemental copper as copper carbonate. Since there are differences in the bioavailability of these two sources of copper for the animal involved, a correction factor should also be used so that the same amount of available copper is supplied to the animal. However, it may be more cost effective and as sound nutritionally to supply a mineral that is 50% available at twice the level as it is to supply a mineral that is 100% available.

Mineral chelates and complexes. A number of mineral chelates and complexes are available. Excellent reviews on the significance of chelates and complexes for the feed industry have been prepared (Nelson, 1988; Kincaid, 1989; Patton, 1990; Spears, 1991; Parks and Harmston, 1994; Ammerman et al., 1995). Chelate designates metal complexes in which the metal atom is held in the complex through more than one point of attachment to the ligand (chelating agent), with the metal atom occupying a central position in the complex.

Naturally occurring chelating agents are widely distributed in all living systems in nature including carbohydrates, lipids, amino acids, phosphates (phytic acid), porphyrins (e.g., hemoglobin and chlorophyll) and vitamins (vitamin B₁₂ and ascorbic acid). A number of drugs also appear to function via chelation, including aspirin, penicillin and tetracyclines.

Some chelates are also defined as proteinates or complexes. Examples of the chelate or sequestered products would include potassium, calcium or cobalt amino acid-chelates or complexes, copper proteinate, zinc proteinate, copper polysaccharide complex, cobalt polysaccharide complex and zinc methionine.

The quantitative measure of the affinity of a metal to complex with a ligand is called the formation or stability constant. A ligand with a higher stability constant will displace a metal from a ligand with a lower stability constant. As an example, Vohra and Kratzer (1964) proposed that the ability of a chelating agent to improve zinc availability from isolated soybean protein depended upon the chelating agent having a higher stability constant for the metal than the metal-binding substance in the feed. Therefore, there is a loss of zinc from the soybean protein to the chelating agent by forming a zinc-chelate complex in the digestive tract. The zinc chelate is then absorbed, and the chelate is then replaced by ligands in the body that have a higher stability constant. Therefore, the stability constant values of a useful chelating agent would be intermediate to the values of isolated soybean protein and the complexing agent in the body.

Natural digestion of foods produces numerous ligands that can complex (chelate) with minerals in the diet and facilitate their passage from the lumen of the intestine into the cells of the intestinal wall, where they eventually chelate with natural ligands that transport the minerals throughout the body. Unfortunately, significant quantities of minerals are complexed with ligands that are inefficiently absorbed and, therefore, lost by excretion.

In theory, the introduction of chelated minerals will increase absorption and utilization of the mineral because of a more favorable binding or stability constant. Therefore, in an animal's digestive system, organic trace minerals, those that are bound to an organic ligand such as protein, amino acids or carbohydrates, may be more biologically available than inorganic trace minerals (Parks and Harmston, 1994).

The pH of the digestive system has a great influence on trace mineral availability.

Generally, organic trace minerals are made by mixing a hydrolysate of plant or animal proteins, carbohydrates or amino acids with a soluble salt, usually a sulfate of a trace mineral. The details of the complete process vary with the manufacturer. However, the quality of the starting material, the type of mineral-ligand binding and the stability constant of the resulting compound substantially affect the quality of organic trace mineral products.

Organic trace minerals have high stability and do not react as readily as single ions. These forms do not interact with vitamins and other ions and are effective at low levels. Not only do organic trace minerals not react with vitamins in the digestive tract but do not react with vitamins in premixes (Shurson et al., 1996).

Use of element-specific amino acid complexes reduced vitamin destruction in a vitamin-trace mineral premix compared to inorganic trace mineral sources. If there is high dietary molybdenum, copper in chelated form has an advantage over an inorganic form as it may escape the digestive system complexing among molybdenum, copper and sulfur (Nelson, 1988).

Organic trace mineral products with the highest amount of insoluble protein would probably be the best choice for ruminant feeds (Parks and Harmston, 1994). These products are not as susceptible to degradation by ruminal microbes and other reactions as are products with high protein solubilities.

Spears (1989) showed that zinc-deficient lambs retained zinc from zinc methionine better than from zinc oxide. Herrick (1989) reviewed zinc methionine feeding in four dairy trials and concluded that the zinc complex-treated animals had lower somatic cell counts and higher milk yields than did control cows. Supplementing mature ewes with complexed minerals resulted in higher concentrations of zinc and copper in the liver (Hatfield et al., 2001).

For young calves, there was greater absorption and retention of zinc when administered in the form of zinc methionine and zinc lysine than that when zinc oxide was administered (Kincaid et al., 1997). Zinc in the form of zinc lysine fed to sheep resulted in the highest levels of metallothionein in liver, pancreas and kidney compared to other zinc sources, thus indicating a more bioavailable source of zinc (Rojas et al., 1996).

Weaning weights were higher for zinc and manganese methionine-supplemented calves compared to control or oxide-supplemented calves (Spears and Kegley, 1991).

Organic selenium (selenium-yeast) has been shown to have a higher bioavailability than selenium as selenite or selenate in cattle (Pehrson, 1993; Awadeh et al., 1997, 1998; Ortman et al., 1999; Valle et al., 2003). Organic selenium has improved performance, blood, milk and tissue selenium concentrations, placental transfer and immune response when compared to inorganic sources.

Some studies have shown no benefit from chelated and complexed minerals, but most have shown positive responses compared to inorganic sources. Rojas et al. (1996) suggested that at adequate levels of dietary zinc, bioavailability of supplemental zinc sources may be less important than under conditions of limited dietary zinc or if very high levels of supplemental zinc are fed.

Dietary copper from sulfate and lysine sources had similar results for cattle with adequate copper status (Rabiansky et al., 1999). However, copper lysine at 16 mg of copper per kilogram appeared to be more beneficial for animals that were borderline to deficient in copper status. Organic copper provided a more bioavailable form of supplemental copper than copper sulfate for postpartum first-calf heifers (Yost et al., 2001).

Much more needs to be learned about the selectivity of organic trace minerals, the most effective kind and quantity, their mode of action and their behavior with different species of animals and with varying diets. Dietary requirements for minerals may be greatly reduced by the addition of organic trace elements to animal diets, but it is generally more economical to use higher levels of inorganic mineral sources than the more expensive mineral chelates and complexes. Nevertheless, organic trace minerals are generally of very high bioavailability, and they are particularly attractive for high-producing or stressed animals or for problem areas (e.g., high dietary molybdenum for ruminants) where cheaper inorganic sources are less effective.

Selecting supplements

Typical free-choice mixtures. Free-choice mineral supplements are generally considered only for livestock that do not have access to concentrates, as minerals for those receiving concentrates are generally provided as part of the concentrate mixture. Even though grazing livestock have been found not to balance their mineral needs perfectly when consuming a free-choice mixture, there is usually no other practical way of supplying minerals under grazing conditions.

As a low cost insurance to provide adequate mineral nutrition, "complete" mineral supplements should be available free-choice to grazing livestock (Cunha et al., 1964). A complete mineral mixture usually includes salt, a low-fluoride phosphorus source, calcium, cobalt, copper, manganese, iodine, iron and zinc. Except where selenosis is a problem, most free-choice supplements should contain selenium. For tropical regions with acid soils, manganese and iron could likely be eliminated from the complete mineral mixture unless iron may help overcome effects of parasitism.

Magnesium, potassium, sulfur or additional elements can also be incorporated into a mineral supplement or can be included at a later date as new information suggests a need. Calcium, copper or selenium, when in excess, can be more detrimental to ruminant production than any benefit derived.

Where high-forage molybdenum predominates, three to five times the copper content in mineral mixtures is needed to counteract toxicity (Cunha et al., 1964). As little as 3 ppm (or 0.4% sulfur) has been shown to decrease copper availability to 50%. Thus, the exact level of copper to use in counteracting molybdenum or sulfur antagonism is a complex problem and should be worked out for each area.

The Table lists the characteristics of a "good" (complete or shotgun) mineral supplement (McDowell, 2003).

A number of so-called authorities feel there is no justification for the use of shotgun (complete) free-choice mineral mixtures that are designed to cover a wide range of environments and feeding regimens and to provide insurance against deficiency. These people feel that shotgun mixtures are economically wasteful and can also be harmful. This author is in disagreement with this viewpoint. There is little danger of toxicosis or excessive cost in relation to the high probability of increased production rates for cattle from administering a complete "shotgun" free-choice mineral mixture following the guidelines in the Table.

Copper and selenium added at recommended levels would be the minerals of most concern for toxicity. However, cattle, unlike sheep, are much less sensitive to copper toxicosis, and inorganic forms of selenium (e.g., sodium selenite) are less utilized when administered in excess of the requirements.

In conclusion, it is best to formulate free-choice mixtures on the basis of analyses or other available data. However, when no information on mineral status is known for a given region, a free-choice complete (shotgun) mineral supplement is definitely warranted.

Special free-choice mixtures. An oral magnesium supplement is of value only during occurrences of grass tetany (Allcroft, 1961). Unfortunately, many commercial magnesium-containing, free-choice mineral supplements are often of little value because (1) they contain inadequate amounts of magnesium to protect against tetany during susceptible periods and (2) provision of such supplements to normal animals during non-susceptible periods is useless as a prophylactic measure, because additional magnesium will not provide a depot of readily available magnesium for emergency use. Some producers feed magnesium supplements about a month before the magnesium tetany season to decrease the amount of magnesium needed daily during the susceptible period.

The provision of special high-magnesium mineral blocks or mineral salt mixtures on pasture was more effective in raising blood magnesium levels quickly after the initial drop than was magnesium fertilization (Reid et al., 1976). Various combinations of magnesium oxide with salt, protein supplements, molasses, other concentrate ingredients and other feeds have been used to obtain optimal magnesium intakes (Miller, 1979).

From West Virginia, average consumption of magnesium by beef cows given a free-choice mixture of 40% salt, 40% dicalcium phosphate and 20% magnesium oxide ranged from 1.3 to 4.2 g per head per day (Reid et al., 1976). This compared to an intake level of 5-10 g magnesium from a similar mixture containing 20% dried molasses or 4.1-8.8 g magnesium from commercial molasses-magnesium oxide blocks (15% magnesium).

Several relatively successful free-choice consumption formulas of both liquid and dry supplements are as follows: (1) magnesium oxide plus molasses at a ratio of 1:1; (2) 97% molasses plus 3% magnesium chloride (often with urea and a source of phosphorus); (3) equal parts of magnesium oxide, salt, bone meal and grain, and (4) a 1:1 ratio of salt and magnesium oxide. In the southeastern U.S., a complete mineral mixture with 25% magnesium oxide (14% magnesium) has been effective in preventing grass tetany in beef cattle (Cunha, 1983). Licking wheels or licking belts are sometimes used to slowly dispense magnesium oxide or magnesium sulfate in molasses.

Other methods of tetany control, including administration of magnesium through fertilizer, foliar application, enemas, water and injections, have been reviewed (McDowell, 1997). Berger (1992) calculated that, in a 100-cow herd, preventing the loss of a single cow every three years from grass tetany would more than pay the cost of magnesium supplementation.

Often young forages contain high concentrations of potassium, with potassium level an important risk factor in the development of tetany. Potassium decreases magnesium absorption (Schonewille et al., 1999). However, there are some conditions where cattle need supplemental potassium. Generally, forages contain considerably more potassium than required by cattle. However, mature pastures that have weathered or hay that has been exposed to rain and sun or was overly mature when harvested can have potassium levels less than adequate for good nutrition (Karn and Clanton, 1977; McDowell, 1985). Potassium is a very soluble element, and dead material that is allowed to leach will have a reduced potassium content.

Even though mature forages are low in potassium, deficiency does not occur if ruminants are provided a molasses-urea supplement in the winter or dry season. Molasses counteracts low forage potassium, as it has a high potassium level (about 4.0%). When molasses is too expensive for ruminant livestock supplementation, the chances of potassium deficiency are greatly increased.

Most often, sulfur is not included in free-choice supplements. Sulfur supplementation will most likely be needed to meet the requirements of ruminants when poor-quality roughages grown on sulfur-deficient soils or feeds combined with urea are fed. There is no sulfur in urea; therefore, the element may need to be added when high levels of urea are fed. Pasture fertilization programs have changed in recent years from using a source of sulfur in single-superphosphate (about 12% sulfur) to triple-superphosphate and other high-analysis fertilizers that contain little or no sulfur.

In a review (Miles and McDowell, 1983) that summarized four cattle supplementation trials, control diets contained between 0.04 and 0.10% sulfur. Intake by sulfur-supplemented cattle increased from 7 to 260%, and production of milk and meat increased from 6% to more than 400%. Some reports from tropical regions indicate that sulfur fertilization may increase forage intake by improving palatability of less-palatable species.

On the contrary, some forages contain high sulfur (>0.4%), which significantly reduces ruminant copper status (Tiffany et al., 2000). High dietary sulfur reduces copper absorption, possibly due to unabsorbable copper sulfide formation, independent from its part in thiomolybdate complexes (Underwood and Suttle, 1999).

Special calcium and phosphorus supplementation is required for high-producing dairy cows to prevent parturient paresis. Parturient paresis can be prevented effectively by feeding a prepartum diet low in calcium and adequate in phosphorus. Prepartal low-calcium diets are associated with increased plasma parathyroid hormone (PTH) and 1,25-(OH)2D (active form of vitamin D) concentrations during the prepartal period. The increased PTH and 1,25-(OH)2D concentrations result in "prepared" and effective intestinal and bone calcium homeostatic mechanisms at parturition that prevented parturient paresis.

Anion-cation balance of prepartum diets (sometimes referred to as acidity or alkalinity of a diet) can also influence the incidence of milk fever (Gaynor et al., 1989; Horst et al., 1997; Vagnon and Oetezel, 1998; Pehrson et al., 1999). Diets high in cations, especially sodium and potassium, tend to induce milk fever, but those high in anions, primarily chloride and sulfur, can prevent milk fever. The incidence of milk fever depended on the abundance of the cations sodium and potassium relative to the anions chloride and sulfate. This concept is now generally referred to as the cation-anion difference (CAD).

Because most legumes and grasses are high in potassium, many of the commonly used prepartum diets are alkaline. There are large variations in the mineral content of roughages fed on different farms, and the mineral content of grass and, consequently, the CAD of a diet can be significantly altered by different types of fertilization (Pehrson et al., 1999).

Addition of anions to a prepartal diet is thought to induce in the cow a metabolic acidosis, which facilitates bone calcium resorption and intestinal calcium absorption (Horst et al., 1997). Diets higher in anions increase osteoclastic bone resorption and synthesis of 1,25-(OH)2D in cows (Goff et al., 1991). Anionic diets may work by increasing target tissue responsiveness to calcitropic hormones. Several options exist regarding methods for the control of milk fever (Horst et al., 1997).

Optimum mineral allowances

Most nutritionists consider National Research Council (NRC) requirements sufficient to prevent clinical deficiency; they may be adjusted upward if a higher level of minerals is needed. Allowances of minerals are those total levels from all sources fed to compensate for factors influencing mineral needs of animals.

These "influencing factors" include those that may lead to inadequate levels of dietary minerals. Mineral levels vary in feed ingredients because of crop location, fertilization, plant genetics, plant disease and weather. For example, corn and other grains will vary in their concentration of minerals depending on varieties, soils, levels of fertilization, maturity at harvest, processing methods and storage. Intensive cropping practices and use of new crop varieties may result in reduced levels of certain minerals in many feedstuffs.

In forage crops, factors that favor production of lush, green plants also favor production of higher levels of minerals, as most plants decline in their mineral content as they mature. Harvesting conditions, processing and storage will affect the mineral content of both feedstuffs and mineral supplements. Minerals are lost from volatilization (particularly iodine and selenium) and leaching from mineral boxes not properly protected from inclement weather.

Bioavailability of mineral supplement information is available but extremely limited for typical feedstuffs. Mineral concentration in diets for livestock receiving concentrate diets is lower than in the past because computerized, least-cost formulations have eliminated feeds high in minerals (also vitamins). Most minerals are not entered as specifications in computer formulations.

Therefore, mineral-rich feedstuffs, such as alfalfa, distiller's solubles or grains, brewer's grains and meat, milk and fish byproducts, are often excluded or reduced when least-cost formulations are computed. The resulting least-cost diet consisting of a grain and oilseed meal (e.g., soybean meal) is usually lower in minerals than a more complex one containing more costly mineral and vitamin-rich feeds.

Additional influencing factors may result in inadequate minerals in animal diets compared to actual requirements and relate to the following:

Physiological makeup and production function. Mineral needs of ruminant animals depend greatly on their physiological makeup, age, health, nutritional status and function, such as producing meat, milk or developing a fetus. For example, dairy cows producing greater volumes of milk have higher mineral requirements than dry cows or cows producing low quantities.

Different breeds and strains of animals vary in their mineral requirements and those of new strains developed for improved production may be higher.

Confinement rearing without access to pasture. Moving of livestock operations into complete confinement without access to pasture has had a profound effect on mineral, as well as vitamin, nutrition (McDowell, 2000). Young, lush, green grasses or legumes are good sources of many minerals. Confinement rearing, including poultry in cages and swine on slatted floors, results in limited animal access to feces (coprophagy), which provide some minerals. Confinement rearing requires producers to pay more attention to higher mineral requirements (Cunha, 1987).

Stress, disease or adverse environmental conditions. Intensified production increases stress and subclinical-level disease conditions because of higher densities of animals. Nutrient levels that are adequate for growth, feed efficiency, gestation and lactation may not be adequate for normal immunity and for maximizing the animal's resistance to disease (Cunha, 1985). Diseases or parasites affecting the gastrointestinal tract reduce intestinal absorption of minerals. If they cause diarrhea or vomiting, this will also decrease intestinal absorption and increase needs.

Mineral interrelationships. Mineral interrelationships are very important in determining mineral requirements. Minerals and other nutrients, such as vitamins, amino acids and energy, are interrelated to some extent. This means there is a correct level for each nutrient in relation to the level of all other nutrients to obtain the best response. Realistically, until mineral interrelationships are understood, only an approximation of mineral requirements can be made.

Body mineral reserves. Body storage of minerals from previous intake will affect daily requirements. The body has large stores of some minerals if liberal dietary intakes were previously available (e.g., calcium and phosphorus in osteous tissue) and can store considerable quantities of trace minerals such as copper, selenium and iodine.

Optimum mineral allowances are needed to permit animals to achieve their full genetic potential for optimum performance. The higher the allowance, the greater is the extent to which it may compensate for the influencing factors that result in higher true requirements; thus mineral allowances higher than NRC requirements may be needed to allow optimum performance.

It should be emphasized that subacute deficiencies can exist although clinical deficiency signs do not appear. Such borderline deficiencies are both the most costly and the most difficult to manage and often go unnoticed and unrectified, yet they may result in poor and expensive gains, impaired reproduction or depressed production. Under farm conditions, there is usually not a single mineral deficiency found, but deficiencies are usually a combination of factors; often deficiency signs will not be clear-cut.

If the NRC minimum requirement for a mineral is the level that barely prevents clinical deficiency signs, then this level moves in relationship to the level required for optimum production responses. Optimum animal performance required under modern commercial conditions cannot be obtained by fortifying diets to just meet minimum requirements. Adequate margins of safety must provide for those factors that may increase certain dietary mineral requirements and for variability in availability within individual feed ingredients.

The NRC requirements often do not take into account that in disease conditions, certain minerals are needed at higher-than-recommended levels needed for response. Minerals play a major role in the immune response, the body's defense system against infectious disease; mineral supplementation above requirements is required for optimum immune responses (Cunha, 1985). Magnesium, phosphorus, sodium, chloride, zinc, copper, iron and selenium have been shown to improve an animal's ability to cope with infections.

Summary

Proper mineral supplementation of grazing livestock is essential for maximizing production. Mineral needs will vary considerably, depending on the many factors discussed herein. The mineral elements most likely to be lacking under tropical conditions are calcium, phosphorus, sodium, cobalt, copper, iodine, selenium and zinc. In some regions, under specific conditions, magnesium, potassium, iron and manganese may be deficient and excesses of fluorine, molybdenum and selenium can be detrimental in a number of regions.

Methods of mineral supplement evaluation and procedures for supplement formulation are discussed. Responsible firms that manufacture and sell high-quality mineral supplements provide a great service to livestock producers.

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